The present invention relates to a device for transporting printing plates stacked in a holder to an exposure and developing station, and for positioning them therein.
Imaged printing plates which can be readily used for printing are prepared by first charging the printing plates electrostatically and then projecting the image of an original onto the individual printing plates. After termination of this exposure step, the printing plate is developed by using a developer, fixed and decoated, whereupon it is ready for use in printing.
From U.S. Pat. No. 4,006,984 an apparatus for the manufacture of printing forms by electrophotographic means is known. In this apparatus, the individual printing forms are removed from a stack in a supply magazine by a transport device which comprises a carriage having a vacuum suction device and which conveys the printing forms to an exposure platform. A corona charging station for the electrostatic charging of the printing plates is installed at the front side of the carriage and extends perpendicularly to the direction of movement of the carriage in order to combine the charging of the individual printing forms with their transport to the exposure platform and thus to save time. The carriage runs on two guide rails and is driven by a motor installed on its upper side, through a gear and a rack-pinion gear arranged in parallel to the direction of movement of the carriage. The carriage has a vacuum plate attached to its underside which is connected by a number of openings with a vacuum pump. When the carriage touches the upper surface of the uppermost printing plate in the plate holder, a reduced pressure is created by which this uppermost printing plate is drawn to the vacuum plate. Then the carriage is moved in the direction of the exposure platform by means of the motor. As soon as it has reached its position above the exposure platform, the carriage is lowered onto the platform and the reduced pressure created in the vacuum plate is interrupted, so that the printing plate is released and deposited on the exposure platform. The latter is also constructed as a vacuum plate, and a reduced pressure is applied to it, so that the printing plate is firmly urged against the exposure platform.
In this known apparatus, the printing plates are stacked in the plate holder with their photoconductive layers pointing in the upward direction, and their coated surfaces are drawn to the vacuum plate of the carriage. When a new stack is inserted into the plate holder or when the apparatus is opened, the incident light may lead to a pre-exposure of those plates which are still in the holder, whereby the printing quality of the readily developed plates may be adversely affected. When the individual printing plates are lifted off the stack, the photoconductive surfaces of the printing plates are touched by the suction devices. It is true that, compared with rollers or conveyor belts which are installed in other known apparatus, a more careful transport of the printing plates is made possible by these suction devices, but nevertheless the formation of mechanical or physical voids on the developed printing plates cannot be completely avoided.
U.S. Pat. No. 4,149,798 discloses a transport station for printing plates which comprises a stacking area where a supply of printing plates is stored. The individual printing plates are separated from one another by sheets of paper for which a disposal area is provided. By means of a control mechanism which is provided with a number of suction cups, the printing plates are transported from the stacking area to a conveyor belt. The control mechanism is pivotally fastened to a guide arm which, via a gear, is connected to a motor, in order to be able to carry out a lateral back-and-forth sliding movement. The uppermost printing plate is engaged by at least four suction cups. Then the control mechanism swings in the upward direction, and simultaneously the guide arm is mechanically moved in the direction of the conveyor belt. Upon release of the vacuum in the suction cups, the control mechanism moves downwardly, and the printing plate is deposited on the conveyor belt. A second control mechanism above the interposed sheet of paper then picks up the sheet and conveys it to the paper disposal bin.
By means of the conveyor belt, the printing plate is transported to an exposure platform. The surface of the exposure platform is provided with a number of holes and forms a chamber which, via a suction line, is connected with a vacuum plate. When the printing plate has been brought into proper alignment on the surface of the exposure platform, it is retained on the latter by creating a vacuum. The printing form is exposed by means of an exposure system comprising a movable carriage platform on which a corona-charging device and a light-reflecting mirror are arranged. A laser for reflecting modulated laser light is installed such that the light beam emitted by the laser is deflected by the mirror and strikes the printing plate on the exposure platform in a plane extending substantially perpendicularly to the surface of the printing plate.
In this known apparatus an accurate positioning of the individual printing plates can be carried out neither in their transverse nor in their longitudinal direction prior to conveying them to the exposure platform and to the other treatment stations, since no means are provided for this purpose. The removal of the separating sheets of paper between the individual printing plates prior to transport to the exposure station requires additional time.